How to tell if a fluorescent ballast is going bad?

Written by maxwell payne
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Most fluorescent lamps contain a ballast, usually electronic, to start up the light and provide sufficient power to warm up the fluorescent bulbs. Types of ballasts include standard ballasts, rapid start ballasts, and instant on ballasts. Depending on the type used, the lights may come on full power right away or take a minute to reach full power.

Over time, the ballasts can begin to go bad. Being aware of the warning signs of a bad ballast can help you identify failing ballasts and replace them before they blow out completely (See Reference 1).

Skill level:


  1. 1

    Ensure that the temperature around the lights is not too hot or cold. Ballasts and their fixtures should not be blocked or covered, as this can cause them to overheat and shut off. At the same time, cold temperatures can prevent the ballast from warming up or can cause the bulbs to reach only partial brightness. Special cold-start ballasts are available for lights that will be used in constantly cold conditions (See Reference 1).

  2. 2

    Listen to the buzzing or the humming of the light. Most fluorescent tube lighting fixtures make a faint humming or buzzing sound as they warm up and run. If the sound is louder than usual, this is a key sign that the ballast is beginning to go bad. Keep in mind that the sound may be slightly louder upon first switching on the lights; you want to listen for louder than normal buzzing after the light has been on for a while (See Reference 1).

  3. 3

    Take notice to see if the lights are turning off and on during normal use. If they completely shut off before coming back on, this may indicate a bad ballast. Ballasts automatically shut off when they overheat; a ballast that does it often, even after adjusting the room temperature cooler, has become damaged from previous exposure to high heat. (See Reference 1).

Tips and warnings

  • Do not turn ballast controlled lights off and on rapidly; they are designed to warm up and stay on for extended periods of time. Rapid switching can shorten ballast life.
  • Always disconnect the light fixture from the power supply before touching the lights or ballast setups.

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